Contents

Soft Matter Keywords

microfluidic, bubbles, laminar mixing, Peclet

Figure 1. (a) Two fluid streams remain nearly unmixed when bubbles are not present. (b) Introducing bubbles along with fluid streams causes the two fluids to fold into one another, aiding in diffusive mixing. Intensity as a function of position across the channel at the specified locations are shown.

Figure 1. (a) Recirculation rolls between slugs of air. (b)-(d) Schematics indicating channel design and the paths slugs take through the branches. As a slug enters one arm, the hydrodynamic resistance in the arm increases, cause the next slug to enter the other arm of the branch.

Figure 1. (a) Two fluid streams remain nearly unmixed when bubbles are not present. (b) Introducing bubbles along with fluid streams causes the two fluids to fold into one another, aiding in diffusive mixing. Intensity as a function of position across the channel at the specified locations are shown.

Summary

This paper details experimental work and simple supporting theory regarding mixing in microfluidic channels. For most microfluidic systems, the Reynolds number remains small (less than 1000), so turbulence is absent and mixing only occurs via diffusion. Typical Peclet numbers in microfluidic channels are on the order of 1e5, indicating that mixing to homogeneity requires length scales on the order of 10 meters. These lengths are not easily achieved on microfluidic devices due to finite substrate limits for fabrication and large pressure drops in the long channels, so the authors propose a novel method of mixing that aid the diffusion process. Using bubbles to fold two liquid streams into one another, greater contact area between the fluids is created, aiding in diffusion.

Practical Application of Research

A drawback to continuous flow microfluidics has been the inability to homogeneously mix streams on-chip. This limits the application of continuous flow microfluidics, as it is difficult to introduce additional reagents or samples to a flow. This passive, on-chip mixing scheme open a new realm of experiments that can take advantage of introduction of precise amounts of fluid at a given spatial or temporal point in a flow.